config: Don't error if tiling_exception_custom is not present

As noted in pop-os/cosmic-comp#1603, there's no
need to a system default config file for this particular setting. So as
an alternative to that PR, we can ensure this isn't printed as an error.

As noted in the description of
pop-os/libcosmic#948, if the system default is
not present, `Error::GetKey` is returned, without mapping to the
`Error::NotFound` variant. So check for that, as well as `.is_err()`.

Author: ids1024

config/src/window_rules/mod.rs

@@ -77,7 +77,15 @@ pub fn tiling_exceptions(context: &cosmic_config::Config) -> Vec<ApplicationExce
     let custom = context
         .get::<Vec<PreciseApplicationException>>("tiling_exception_custom")
         .unwrap_or_else(|why| {
-            tracing::error!("tiling exceptions custom config error: {why:?}");
+            if why.is_err() {
+                if let cosmic_config::Error::GetKey(_, err) = &why {
+                    if err.kind() != std::io::ErrorKind::NotFound {
+                        tracing::error!("tiling exceptions custom config error: {why}");
+                        return Vec::new();
+                    }
+                }
+            }
+            tracing::debug!("tiling exceptions custom config not present: {why}");
             Vec::new()
         });